1. Field of the Invention
The invention relates to a solenoid valve that controls a pressure on a working fluid according to a current supplied to a coil.
2. Description of Related Art
A solenoid valve is conventionally known which includes a solenoid portion generating an electromagnetic force, a tubular sleeve with a valve hole, and a shaft-like spool moving through the valve hole in an axial direction and in which a pressure on a working fluid is controlled by the movement of the spool in the axial direction (see, for example, Japanese Patent Application Publication No. 2014-105726 (JP 2014-105726 A).
The solenoid valve described in JP 2014-105726 A has a solenoid portion with a plunger that moves in the axial direction according to a current supplied to a solenoid coil, a sleeve shaped like an inner tube and disposed coaxially with the plunger, and a spool that is housed in a valve hole formed in the sleeve and that slides on an inner surface of the sleeve in conjunction with the movement of the plunger in the axial direction.
The solenoid portion has a solenoid coil, a tubular cover member (solenoid case) made of a magnetic material supporting the solenoid coil, a core member (solenoid core) made of a magnetic material, and a plunger that moves in the axial direction with respect to the cover member. The core member has, in a central portion thereof, a through-hole that penetrates the core member in the axial direction, and a shaft is inserted through the through-hole. At an end of the core member that is on the opposite side to a sleeve, a cylindrical yoke portion having a recess is provided. One end of the plunger is fitted in the recess.
When a current is passed through the solenoid coil, the plunger is attracted by a resultant electromagnetic force toward a bottom surface of the recess of the cylindrical yoke portion of the core member. A slight gap (air gap) is formed between an inner peripheral surface of the recess and an outer peripheral surface of the plunger. A magnetic flux generated in the solenoid coil circles around the solenoid coil along a magnetic path including this air gap.
Under the magnetic force of the solenoid coil, the plunger moves forward and backward in the axial direction along with the shaft to press the sloop via the shaft, thus moving the spool in the valve hole. Consequently, a channel for a working fluid in the valve hole is switched to change a pressure under which the working fluid is fed to a control target apparatus.
The shaft is supported by a first bearing bushing disposed in the through-hole in the core member such that the shaft is movable in the axial direction. The plunger is supported by a second bearing bushing disposed inside the cover member such that the plunger is movable in the axial direction.
In the solenoid valve configured as described above, slight gaps need to be formed between the first bearing bushing and the shaft and between the second bearing bushing and the plunger in order to allow the plunger and the shaft to move smoothly in the axial direction. Large gaps may increase backlash of the plunger and the shaft with respect to the core member and the cover member to vary the width of the air gap between the inner peripheral surface of the recess of the core member and the outer peripheral surface of the plunger. A pressing force according to the current supplied to the solenoid coil may not be applied to the spool. On the other hand, excessively small gaps between the first bearing bushing and the shaft and between the second bearing bushing and the plunger may hinder the plunger and the shaft from sliding smoothly.
In the solenoid valve described in JP 2014-105726, inside the core member and the cover member, the first bearing bushing and the second bearing bushing are disposed in the magnetic path for the magnetic flux generated in the solenoid coil. This reduces magnetic efficiency. Furthermore, in addition to machining errors in the core member and the cover member, dimensional errors in the first and second bushing bearings in a radial direction affect the sizes of the gaps between the first bearing bushing and the shaft and between the second bearing bushing and the plunger. Thus, to allow the plunger and the shaft to move smoothly in the axial direction, the backlash of the plunger and the shaft with respect to the core member and the cover member needs to be permitted, and large air gaps accordingly need to be set between the inner peripheral surface of the recess of the core member and the outer peripheral surface of the plunger.
However, setting large air gaps between the inner peripheral surface of the recess of the core member and the outer peripheral surface of the plunger increases magnetic resistance in the magnetic path for the magnetic flux generated in the solenoid coil, which reduces magnetic efficiency. Thus, the magnetic force that may be generated in the solenoid coil needs to be strengthened by, for example, increasing the number of turns of a winding of the solenoid coil. This is conventionally a restriction on reducing the size of the solenoid valve.